Praising him as a tireless public servant who obsesses about US security, he added: "I'm not sure he has slept in four years".

White House colleagues have described him as "a priest whose blessing has become indispensable" to the president in their selection of targets for drone strikes from a so-called "kill list".

A career spy who served as CIA station chief in Saudi Arabia, Mr Brennan was also accused of being complicit in the agency's torture of terror suspects under former president George W. Bush.

Controversy over his involvement in the so-called "enhanced interrogation techniques" is believed to have prevented Mr Obama from nominating Mr Brennan to lead the CIA after his 2008 election win.

He promised to ensure the work of the CIA "always reflects the liberties, the freedoms, and the values that we all hold dear".

Mr Obama also confirmed his nomination of Chuck Hagel, a former Republican senator whose foreign policy stances have attracted criticism from former colleagues, as his next defence secretary.

If confirmed by the Senate, the 66-year-old would be the first combat veteran of the Vietnam War, and the first veteran of enlisted rank, to head the defence department.

Hailing him as an "American patriot" who "bears the scars and the shrapnel of battles fought in our name", Mr Obama said: "Chuck knows that war is not an abstraction".

The selection of Mr Hagel, who once criticised intimidation of US politicians by "the Jewish lobby" has been sharply criticised as an insult to Israel that could jeopardise a key alliance.

Mr Obama on Monday praised Mr Hagel's "willingness to speak his mind", stating that he "understands that America stands strongest when we stand with allies and with friends".

Lindsey Graham, a Republican Senator for South Carolina, has said Mr Hagel "would be the most antagonistic secretary of defence towards the state of Israel in our nation's history".

Mr Hagel has also been criticised for opposing additional unilateral sanctions on Iran and for urging Israel's government to engage in direct talks with Hamas, the militant Islamic group.

He pledged yesterday to give Mr Obama his "honest and most informed counsel".

Both nominees must be confirmed in votes by the US Senate, where Mr Obama's Democrats hold a majority. So far Ted Cruz, a Texas senator, has promised to vote against Mr Hagel's appointment.

Coming soon after the selection of Senator John Kerry to replace Hillary Clinton as US Secretary of State, the nominations also prompted accusations that Mr Obama had failed to make his Cabinet diverse.

Tim Miller, a Republican spokesman, said Mr Obama's promotion of "three old white guys" who backed the war in Iraq made a mockery of his re-election campaign slogan, which he restyled as "Forward-ish".

Mr Brennan was also blamed for a series of erroneous briefings to the media following the death of Osama bin Laden in May 2011, which angered the then-defence secretary Robert Gates.

After Mr Brennan said bin Laden had been killed while using one of his wives as a shield, officials later said that this was untrue.